Metal matrix compositions and method of manufacturing thereof

ABSTRACT

An improved metal matrix composite which, in a preferred embodiment disclosed herein, utilizes boron carbide as the ceramic additive to a base material metal. The metal matrix composite of the present invention begins with the preparation of the boron carbide powder by particle size selection in a jet mill. The resulting powder and metal powder are then mixed by blending of powder of all the various elements such as by means of a conventional blender to uniformly mix powdered substances and avoid stratification and settling. After the particles have been sufficiently mixed, they are degassed and then placed into a die and then into a cylindrical container where the particulates are subjected to extremely high pressures transforming the elements into a solid ingot. It is from these ingots that the extrusion tubes or other articles of manufacture may then be made.

This is a division of application Ser. No. 08/183,728 filed Jan. 19,1994 now U.S. Pat. No. 5,486,223.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to metal matrix compositions.Such composites comprise one or more base material metals such as forexample, aluminum, titanium or magnesium, to which is added a selectedpercentage of ceramic materials which alter the properties of the basematerial metal in a positive manner. Strength, hardness and drawabilityare increased. Drawability facilitates fabrication of various articlesof manufacture from such composite materials. More specifically, thepresent invention pertains to an improved metal matrix composite which,in a preferred embodiment, uses boron carbide as the added ceramicmaterial. The composites result from a novel method of manufactureproducing a composite which is lighter, stronger, stiffer and which hasa higher fatigue strength than other available alloys of the basematerial metal and which is also lighter, stronger, stiffer and whichhas a higher fatigue strength than prior art metal matrixes, compositesand particularly those metal matrix composites which are of comparablecost.

2. Prior Art

In recent years metal matrix compositions or composites have becomepopular materials for a variety of applications. This new family ofmaterials has become popular because of improvements in stiffness,strength and wear properties. Basic metal matrix composites are madetypically with aluminum, titanium or magnesium as the base materialmetal. Then certain percentages of ceramics are added. Typical ceramicsare boron carbide, silicon carbide, titanium diboride, titanium carbide,aluminum oxide and silicon nitride. Most known metal matrix compositesare made by introducing the ceramics into the molten metal. In largeproduction runs of metal matrix composites, the ceramic reinforcementmust be wetted by the liquid metal to facilitate incorporation of thereinforcement into the melt. In those metal matrix composites usingsilicon carbide and aluminum, the silicon carbide is thermodynamicallyunstable in molten aluminum which leads to the formation of aluminumcarbide at the interface and increased concentration of silicon in thematerial matrix during the solidification process. This interfacereaction is believed to have detrimental effects on the mechanicalproperties of the resulting composite by reducing the interface strengthand changing the composition.

Recently, powder metalurgy consolidation has emerged as a competingmethod of fabricating metal matrix composites by consolidating thepowders by means of hot pressing and conventional powder metalurgyoperations with vacuum sintering to achieve a high density green body.By following certain isopressing and sintering techniques, a 99%theoretical dense billet can be achieved.

In the present invention it has been found that the most desirableceramic candidate for metal matrix composites is boron carbide. Boroncarbide is the third hardest material known and the hardest materialproduced in tonage. Boron carbide powders can be formed by a variety ofreactions including the carbon reduction of any of several boron-oxygencompounds including boric oxide, boric acid, borax, boracite as well asby the direct combination of the elements. Usually most commercial boroncarbide is produced in arc furnaces. Boric acid is added together withcarbon in the form of coke and heated to very high temperatures. Anelectric arc is maintained between graphite electrodes inside a furnace.The synthesis reaction is accompanied by the release of large volumes ofcarbon monoxide. Venting and disposal of the carbon monoxide gasconstitutes a major design consideration. Boron carbide is also thelightest Of all of the ceramics typically used in metal matrix compositetechnology, but it is very hard and expensive. Its hardness limits itsextrudability. Thus it would be highly advantageous if it were possibleto produce an improved metal matrix composite which utilizes an advancedceramic such as boron carbide but which, unlike the prior art, resultsin an extrudable composite material which allows easy fabrication ofvarious articles of manufacture so that such resulting articles have thespecific strength and stiffness improvements as compared to equivalentarticles of manufacture using only the base material metals.

SUMMARY OF THE INVENTION

The present invention comprises an improved metal matrix compositewhich, in a preferred embodiment disclosed herein, utilizes boroncarbide as the ceramic additive to a base material metal. Thefabrication process is unlike that of a number of other metal matrixcomposites because it is not made through molten processes. Morespecifically, instead of melting the boron carbide with the aluminum,nickel, zinc, titanium or other base material metal, the metal matrixcomposite of the present invention begins with the blending of powder ofall the various elements such as by means of a jet mill which isbasically an air blaster used to uniformly mix powdered substances andavoid stratification and settling. After the particles have beensufficiently mixed, they are directed into a die and then into acylindrical container where the particulates are subjected to extremelyhigh pressures transforming the elements into a solid ingot. It is fromthese ingots that the extrusion tubes or other articles of manufacturemay then be made. The resulting advanced metal matrix composite is inthe boron carbide embodiment of the invention, 60% lighter, 30%stronger, 40-45% stiffer and 50% higher in fatigue strength than any ofthe top of the line 7000 series aluminum alloy materials. In addition,the inventive material is 7-8% lighter, 26% stronger, 5% stiffer, andhas 35-40% greater fatigue strength than most popular metal matrixcomposites available in the prior art.

In the preferred embodiment disclosed herein the base material metal ispreferably an aluminum alloy or titanium alloy provided in powder formand preferably being approximately 97% pure with the balance of thematerial comprising various trace metals such as chromium, copper, iron,magnesium, silicon, titanium and zinc. The boron carbide powder ispreferably 99.5% pure boron carbide having a particulate size in therange of 2-19 microns with a mean or average size of approximately 8.4microns. In one typical embodiment of the invention, the metal basematerial was selected from an aluminum alloy 6061T-6 to which was addedapproximately 12% by weight, the aforementioned boron carbide powder towhich was added silicon in an amount of 0.1-0.4%, iron in the amount of0.05-0.4% and aluminum in an amount of 0.05-0.4%. The underlying boroncarbide material was approximately 77% boron content and 22% carboncontent. A metal matrix composite made from the aforementioned materialsin accordance with the fabrication process of the present invention tobe described hereinafter, resulted in a composite material whichexhibited an ultimate tensile strength of 70.1, a yield strength of61.2, and a drawability factor of 71.9 on a scale of 0-100. Furthermore,the resulting material is approximately as hard as chromoly steel buthas a density which is even lower than aluminum alloy. Importantly, thematerial of the present invention is readily extrudable. In a preferredextrusion step, ingots of the metal matrix composites of the presentinvention are extruded through a titanium diboride die bearing materialwhich exhibits a significant increase in die insert life. Furthermore,the present invention is readily weldable. In fact, the coated boroncarbide particulates of the material disclosed herein tend to flux andmove into the weld pool which creates a very strong weld joint. Thus thepresent invention is not only highly suited for the manufacture ofvarious shaped articles, but is also suited for interconnecting sucharticles by conventional welding processes as will be hereinafter morefully explained.

OBJECTS OF THE INVENTION

It is therefore a principal object of the present invention to providean improved metal matrix composite material which exhibits certainadvantageous properties and manufacturability conducive to thefabrication of certain articles of manufacture having improvedcharacteristics such as reduced weight, higher strength and increasedhardness.

It is an additional object of the present invention to provide animproved metal matrix composite material which is especially adapted foruse as structural members in lightweight applications such as bicycleframes and the like while retaining or improving the strength andhardness at the same relative cost of comparable materials used insimilar structures.

It is still an additional object of the present invention to provide ametal matrix composite material which is stiffer and lighter thanaluminum while being as hard as steel and extremely fracture resistantwhile also being extrudable and weldable, thus permitting thefabrication of extremely high strength, lightweight structural membersat reasonable cost.

It is still an additional object of the present invention to provide amethod for manufacturing an improved metal matrix composite material toresult in a material having superior hardness, strength and densitycharacteristics while being extrudable and weldable for use in themanufacture of a variety of structural members which may be readilyconnected to one another such as in bicycle frames, aircraft parts,tooling, sporting equipment, eyewear, automotive parts, electronicparts, furniture and medical equipment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The preferred embodiment of the present invention uses aluminum alloy asa base material metal and boron carbide as the added ceramic material.In the preferred embodiment of manufacture the aluminum alloy isprovided in the form of a metal powder which is blended with jet milledboron carbide particulates that have been processed and have certainchemical and particulate size attributes. The boron carbide ispreferably at least 99.5% pure and has a 2-19 micron particle size withan average particle size of about 8.4 microns. Included in the boroncarbide powder is 0.1-0.4% silicon, 0.05-0.4% iron and 0.05-0.4%aluminum. Trace amounts of magnesium, titanium and calcium may also beprovided. Two exemplary semi-quantitative analyses of acceptable boroncarbide powders for use in the present invention are shown hereinbelowin Tables I and II.

                  TABLE I                                                         ______________________________________                                        B                     77.3%                                                   Si                    0.37                                                    Mg                    0.0016                                                  Fe                    0.026                                                   Al                    0.18                                                    Cu                    0.0021                                                  Ti                    0.0088                                                  Ca                    0.0049                                                  Other elements        nil                                                     C,O.sub.2             (BAL)                                                   ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        B                    77.7%                                                    Si                   0.14                                                     Mg                   0.0017                                                   Fe                   0.074                                                    Al                   0.13                                                     Cu                   ND 0.0002                                                Ti                   0.017                                                    Ca                   0.0048                                                   Other elements       nil                                                      C,O.sub.2            (BAL)                                                    ______________________________________                                    

The addition of small amounts of pure aluminum, silicon and iron to thearc furnace during the production of boron carbide, such as by thereaction of boric acid and carbon, has been found to improve the boroncarbide for use in this metal matrix composite. These metal elements donot go out of solution. They stay in the boron carbide and provide achelating opportunity for the base material aluminum. These additionalmetals form an inter-metallic chemical bond with the main metal alloy.However, it will be understood that the aforementioned additions of purealuminum, silicon and iron, may not be the only metals which can be usedfor the aforementioned purpose. By way of example, virtually any lowtemperature reacting metal that forms an inter-metallic phase below theprocessing temperature of the metal matrix composite ingot, would beuseable in the present invention for the purpose indicated. The typicalrelative weight contributions of the boron carbide powder and basematerial metal powder is 12-15% of the former and 85-88% of the latterdepending upon the specific characteristics desired for the finishedproduct.

After the boron carbide has been jet milled to the selected particulatesize and with the aluminum alloy powder blended together in a doublechamber "V" blender, for two and one-half hours at 20 to 30 RPM in aninert gas, the powders are degassed at 200 degrees Centigrade for onehour in a vacuum of -5 to -8 Torr and then placed in a latex bag andisopressed at 65,000 psi. The isopress bag resembles the shape of theingot that is to be extruded. The latex bag is degassed and clamped off.The maximum pressure is held for at least a one minute soak. Theresulting ingots are removed from the bag and placed into a vacuumfurnace to undergo a sintering cycle in accordance with the followingpreferred embodiment of the process of the present invention.

First, the ingots are heated from room temperature to 300 degreesCentigrade over a twenty minute ramp period during which time binder andwater are burned off. The ingots are then heated to 450 degreesCentigrade over a fifteen minute ramp period during which the remainingbinder is burned off. The ingots are then heated to 625 degreesCentigrade over a forty minute ramp period during which the temperatureincreases accordingly. At 625 degrees Centigrade the ingot is held andsoaked at that temperature for 45 minutes during which close grainboundaries are formed. The ingot is then cooled from 625 degreesCentigrade to 450 degrees Centigrade over a twenty minute period bymeans of a nitrogen gas backfill. Finally, the ingots are cooled to roomtemperature at a rate not faster than 40 degrees Centigrade per minuteagain using nitrogen gas. The ingots are then turned down by a metallathe to bring them into an extruding shape with a typical selectedouter diameter of between 31/2 and 7 inches to a tolerance of 15,000thsof an inch. The ingots are then available for extrusion.

Extruding the metal matrix composite of the present invention firstinvolves preheating the ingots in a resistance furnace for a minimumperiod of one hour at 555 degrees Centigrade. This is normally done intwo steps. First the ingots are heated to 315 degrees Centigrade in aholding furnace and then heated to a higher temperature and held untilthe ingot temperature reaches 555 degrees Centigrade. The ingots arethen loaded directly into a container or chamber from the furnace. Thechamber temperature should preferably be 488 degrees Centigrade. Theface pressure within the chamber depends upon the type of extrusiondimensions that are desired. Typically, the pressures used are 15-20%higher than extrusion pressures used for 6061 aluminum ingots. Forexample, for a 31/2 inch outer diameter billet made of the metal matrixcomposite of the present invention, 3,500 psi peak (break out) pressureis typically used and results in an extruding pressure of about 3,000psi. The speed of the extrusion could be an average of 15-30 feet perminute and the exit temperature should be 20 degrees Centigrade coolerthan the container temperature. The speed of the ram used for theextrusion should run 31/2 inches every minute on a typical 31/2 inchouter diameter ingot.

Although the present invention may be extruded in conventional dies, ithas been found that for maximum die insert life, a die bearing materialmade of titanium diboride is preferred. The titanium diboride diebearing material is preferably hot pressed and then electrodischargemachined to the appropriate size. A small amount of boron carbide may beused to increase the hardness of the die. Typically, the die is made of99.5% pure titanium diboride in an amount equal to 92-98% by weight, theremaining fraction being 99.5% pure boron carbide having particulatesizes less than 10 microns. The hot press cycle for manufacture of thedie bearing material is preferably done at 1,800 degrees Centigradeusing a 3,500 psi pressure with the pressure and temperature maintaineduntil a zero drop in ram travel is obtained.

The extruded metal matrix composite provides the greatest benefit if itis heat treated using a T6-type heat treatment which comprises two hoursat 530 degrees Centigrade with a cold water quench and an artificialaging at 177 degrees Centigrade for ten hours. All welding however hasto be accomplished before heat treatment is applied. Unlike other metalmatrix composites which contain silicon carbide and aluminum oxide wherewelding can be a problem, the metal matrix composite of the presentinvention is readily weldable. Other metal matrix composites formaluminum carbides as brittle components of a weld. Aluminum carbides areformed from the chemical reaction of aluminum and silicon carbide.Because of the surface area of the aluminum oxide particulates and metalmatrixes, clumping and dewetting occurs. These brittle components andparticulates clump together thereby greatly decreasing the strength of aweld body. The metal matrix composite of the present invention does nothave these problems. The coated boron carbide particulates tend to fluxand move into the weld pool which creates a very strong weld joint.Because boron carbide particulates have a melting point of 2,450 degreesCentigrade, the boron carbide is chemically inert at aluminum processingtemperatures.

Depending upon the ratio of boron carbide to aluminum and also dependingupon the particular aluminum alloy used as the base material metal, theresulting material has a density of approximately 2.69 grams per cubiccentimeter which is lower than aluminum 6061. The resulting materialalso has an ultimate strength of from 70-104 ksi, a yield strength of61-98 ksi, and is extremely fracture resistant and more predictable thanother composites. Furthermore, the resulting material of the presentinvention has a hardness which is comparable to that of titanium andchromoly steel, but a density which is roughly a third of steel androughly 60% of titanium.

Two advantageous products made from the metal matrix composite of theinvention are bicycle frames and golf club heads. Bicycle frames madefrom extruded and welded tubing of the inventive material are lighter,stiffer and stronger than comparable bicycle frames made of moreconventional materials such as aluminum, steel or titanium. In golfclubs, the lower density of the inventive material allows for thickerwalled heads, better weight distribution, balance and aerodynamics.Furthermore, a larger "sweet spot" is possible in tournament legalclubs.

Having thus described a preferred embodiment of the material compositionand method of fabrication of the present invention, what is claimedis:
 1. A method of fabricating a metal matrix composite comprising thefollowing steps:a) blending powders of a bore material metal, boroncarbide and at least one metal additive having an inter-metallic phasetemperature below the melting point of said base material metal; whereinsaid boron carbide constitutes about 10% to 16% of the powders by weightand said additive constitutes less than about 1.5% of said powders; b)degassing said blended powders; c) isopressing said blended powders at apressure of at least 65,000 psi; d) heating said isopressed powders upto at least 625 degrees Centigrade over a selected period of time; e)configuring said isopressed and sintered powders to the desired shape;and f) heat treating the resulting shape of step e).
 2. The metal matrixcomposite fabrication process recited in claim 1 wherein step e) isperformed by extruding said isopressed and sintered powders at aselected temperature and pressure through an extrusion die of selecteddimensions.
 3. The metal matrix composite fabrication process recited inclaim 1 wherein said base material metal is taken from the groupconsisting of: aluminum, titanium, alloys of aluminum and alloys oftitanium.
 4. The metal matrix composite fabrication process recited inclaim 1 wherein said at least one metal additive is taken from the groupconsisting of: aluminum, silicon, iron and titanium.
 5. The metal matrixcomposite fabrication process recited in claim 1 wherein said boroncarbide powder has a particulate size in the range of 2 to 19 micronsand an average particulate size of about 6.5 microns.
 6. The metalmatrix composite fabrication process recited in claim 2 wherein saidextrusion die has a liner structure comprising titanium diboride andboron carbide.